Method of fabricating wavelength conversion device

ABSTRACT

Disclosed is a method for fabricating a wavelength conversion device that is capable of suppressing unintended and random polarization reversal due to heating thereby achieving higher wavelength conversion efficiency. The method includes: forming an insulating layer on one place of a crystal substrate naturally and uniformly polarized in a thickness direction; forming an insulating layer pattern with line-and-space by photolithography; then supplying conductive fluid to both planes of the crystal substrate to apply voltage to the crystal substrate, thereby a wavelength conversion device that is periodically polarization-reversed is fabricated. When temperature of the crystal substrate decreases after heating, an ionizer supplies ions to a surface of the crystal substrate, negative ions collect on +z plane, and positive ion collect on −z plane, thereby unintended and random polarization reversal is suppressed.

FIELD OF THE INVENTION

The present invention relates to a method for manufacturing a wavelengthconversion device that may leverage a non-linear optical effect.

DESCRIPTION OF THE RELATED ART

A non-linear optical effect is known as a phenomenon that an inducedpolarization becomes non-proportional to an incident light electricfield when a highly intense (high-intensity) light such as a laser lightis incident (emitted) into an object. As one of this kind of non-linearoptical effect, a typical non-linear optical effect includes a secondharmonic generation that generates a light having half wavelength of anincident wavelength, which is a high prospect as a wavelength conversiontechnique. For example, the technique that converts an infrared lighthaving a wavelength of 1064 nm wavelength emitted from an infraredsemiconductor laser into a green light having a wavelength of 532 nmwavelength, which is half wavelength of the infrared light, is expectedfor practical use in a field of a projector or the like, due to the factthat a development of the laser light source capable of directlyemitting the light in a range of this wavelength band is delayed.

As a device (element) leveraging the non-linear optical effect, thewavelength conversion device (element) is known that generates thesecond harmonic by using the quasi phase matching (QPM) method. The QPMmethod is a wavelength conversion method that leverages a polarizationreversal (polarization-reversed) crystal in which a direction of anatural (intrinsic or spontaneous) polarization of the non-linearoptical crystal having ferroelectric property is alternatively reversedby 180 degree in turn. The QPM method is known as a method that iscapable of attaining higher wavelength conversion efficiency withfavorable beam quality.

More particularly, lithium niobate (LiNbO₃, LN), lithium tantalite(LiTaO₃, LT), KTiOPO₄ (KTP) or the like are exemplarily known as thenon-linear optical crystal. These kinds of crystals, which are thenon-linear optical crystals with periodically polarized structures (PP),are denoted as “PPLN (Periodically poled LiNbO₃)”, “PPLT”, “PPKTP” orthe like, respectively.

Inter alia, the PPNL crystal in which MgO is doped is known as a crystalthat is capable of attaining higher resistance to the light damage(photorefractive damage) with lighter wavelength conversion efficiency.

LISTING OF REFERENCES Patent Literatures

PATENT LITERATURE 1: PCT International Publication No. WO 2011/024392

SUMMARY OF THE INVENTION Problem to be Solved

However, in the field of above mentioned fabrication (manufacture) ofthe wavelength conversion device leveraging the QPM method, it has beenturned out that the higher wavelength conversion efficiency can behardly attained when the fabrication process includes a heating process(step or treatment). Hereinafter, this kind of problem will be explainedbelow.

In order to find out a cause of the above mentioned problem of thewavelength conversion device with lower wavelength conversion efficiencythan expected, the inventor of the present invention applied ahydrofluoric acid treatment on a surface of the wavelength conversiondevice. Applying the hydrofluoric acid treatment causes the surface ofthe wavelength conversion device to be scraped (shaved). The way ofscraping (shaving) differs depending on the direction of thepolarization so that it can visualize the polarization structure onparticular wavelength conversion device.

FIG. 6 shows a result of the hydrofluoric acid treatment to thewavelength conversion device with lower wavelength conversion efficiencythan expected, and also shows a picture of the surface of the wavelengthconversion device to which the hydrofluoric acid treatment is applied.

As shown in FIG. 6, in the fabricated (manufactured) wavelengthconversion device, a plurality of hexagonal patterns of various sizescan be visibly recognized. The patterns show that the wavelengthconversion device has a state that is polarized (polarization occurs oris generated) in the different direction from the circumference. Thepatterns also show the different polarization, which is different fromthe line-and-space (striped) polarization intended to be formed, isgenerated (i.e., unintended polarization reversal (reversedpolarization.))

Although the periodic polarization reversal structure is not shown dueto the limitation on the particular resolution in taking image, it canbe observed that the size of the spot like pattern is much more largerthan the cycle (pitch) of the polarization reversal (A) shown in FIG.1). As a result, the quasi phase matching (QFM) is not accomplished inthe region in which the spot-like pattern is formed (i.e., the region inwhich unintended and random polarization reversal (polarization reversedregion) is generated). Accordingly, the harmonics do not mutuallystrengthen but rather mutually attenuate (counteract) conversely.Certain numbers of regions in which these kinds of unintendedpolarization reversal occur can be observed. Resultantly, it isconsidered that these regions cause the wavelength conversion efficiencyto be reduced.

As will be described below, certain method can be employed thatperiodically applies voltage to intentionally reverse the polarizationin order to obtain the periodic polarization reversal (periodicallypolarization-reversed) structure. If the region with the above spot-likepattern is formed only in the region to which the voltage is applied(i.e., the region in which its polarization is to be intentionallyreversed), then it cause no problem in theory, because the polarizationin those regions should be intrinsically reversed.

However, this kind of case is extremely rare, because the size of thespot like pattern is much larger than the pitch (cycle) of thepolarization reversal (A). Even assuming that the size of the spot likepattern is smaller than the pitch (cycle) of the polarization reversal(A), still the voltage is not applied. This is because, in the case thatthe spot like pattern is (unintentionally) formed in the region in whichthe polarization should not be reversed (i.e., the region in which thenatural (spontaneous) polarization should be kept), this portion iscovered with insulating layer pattern, as shown in FIG. 2. For thisreason, unintentional and random polarization reversal is kept generatedso that the quasi phase matching (QPM) is not accomplished to entailreduction in wavelength conversion efficiency.

According to further research conducted by the inventor of the presentinvention, it has been turned out that this kind of unintentional andrandom polarization reversal occurs due to a heating process for thecrystal substrate, which is carried out during the manufacture(fabrication) of the wavelength conversion device. As will be describedbelow, according to the fabrication method of embodiments of the presentinvention, the method may include a pre-bake process (treatment) and/orpost-bake process (treatment) for the resist. For this reason, thetemperature of the crystal substrate is once elevated to the requiredtemperature for heating and then dropped (decreased or lowered). As thecrystal substrate is ferroelectric and pyroelectric, it is conceivedthat the polarization reversal (reversed polarization) in a randommanner cannot avoid being generated during the change in temperature.Hereinafter throughout the specification, “randomly” or “in a randommanner” means that the generated polarization reversal is not a periodic(regular) polarization reversal, unlike as shown in FIG. 1.

It is presumed that this kind of unintentional and random polarizationreversal is generated (occurs) probably in an area in which a crystaldefect such as a lattice defect exists. In the area in which the crystaldefect exists, much amount of occurrence of the energy levels isobserved so that the area is in a high state in energy intrinsically.For this reason, the polarization can be easily vary due to the changein temperature. Accordingly, it is presumed that the random polarizationreversal, as shown in FIG. 6, cannot avoid being generated.

The present invention is accomplished (developed) in view of these factsand knowledge, and an object of the present invention is to provide amethod of fabricating (manufacturing) a wavelength conversion devicethat is capable of achieving higher efficiency of wavelength conversionby suppressing the unintentional and random polarization reversal.

Solution to the Problem

In order to solve the above mentioned problem, according to a firstaspect of the present invention, there is provided a method offabricating a wavelength conversion device. The wavelength conversiondevice is fabricated from a crystal substrate and has a structure thatis periodically polarization-reversed in the direction perpendicular toa thickness direction of the crystal substrate formed from aferroelectric crystal demonstrating a non-linear optical effect. Thefabrication method comprises: heating the crystal substrate; andremoving electricity on a surface of the crystal substrate whentemperature of the crystal substrate being changing due to the heating.

In order yet to solve the above mentioned problem, according to a secondaspect of the present invention, the removing electricity may be carriedout by collecting, on the surface of the crystal substrate, ions havinga polarity different from a polarity on a region of the surface of thecrystal substrate in which natural polarization occurs.

In order yet to solve the above mentioned problem, according to a thirdaspect of the present invention, the removing electricity may be carriedout by use of an ionizer.

In order yet to solve the above mentioned problem, according to a fourthaspect of the present invention, the method may further comprises:forming an insulating layer on at least one plane of the crystalsubstrate; forming a pattern with line-and-space of the insulatinglayer; and periodically applying voltage to the crystal substrate usingthe formed pattern with line-and-space of the insulating layer, whereinthe heating is carried out after the forming the insulating layer, andbefore or after the forming the pattern.

In order yet to solve the above mentioned problem, according to a fifthaspect of the present invention, the applying voltage may furthercomprises: supplying conductive fluid to the crystal substrate in amanner that the conductive fluid contacts a region of the surface of thecrystal substrate that is not covered with the insulating layer, theregion consisting of each of linear portions constituting theline-and-space; and applying the voltage to the crystal substratethrough the conductive fluid.

In order yet to solve the above mentioned problem, according to a sixthaspect of the present invention, the heating may be carried out afterforming the insulating layer.

Furthermore, in order yet to solve the above mentioned problem,according to a seventh aspect of the present invention, the removingelectricity may be carried out when the temperature of the crystalsubstrate decreases.

Furthermore, in order yet to solve the above mentioned problem,according to an eighth aspect of the present invention, there isprovided a wavelength conversion device fabricated by the abovementioned method of fabricating a wavelength conversion device.

As will be described below, according to the first aspect of the presentinvention, generation of an unintentional and random polarizationreversal may be suppressed when temperature of the crystal substratechanges (is changing) due to heating in the heating step. Accordingly,intentional and periodic polarization structure (polarized structure)can be fabricated (manufactured) in favorable quality. As a result, adesirable wavelength conversion device with higher wavelength conversionefficiency can be obtained.

Further, according to the second aspect of the present invention, inaddition to the above mentioned advantageous effect, the unintentionaland random polarization reversal is suppressed by ion supply. As aresult, the configuration for fabricating the wavelength conversiondevice can avoid being larger scale, and easier adjustment and controlfor fabricating the wavelength conversion device can be attained.

Further, according to the third aspect of the present invention, inaddition to the above mentioned advantageous effect, the polarizationreversals are suppressed on both sides by supplying reversed polarity(reverse polarity) ions from the both sides. As a result, an even highereffect of suppressing the polarization reversal can be obtained.

Further, according to the fourth aspect of the present invention, inaddition to the above mentioned advantageous effects, the unintentionaland random polarization reversal can be suppressed during a pre-baketreatment (process) or post-bake treatment (process) for the insulatinglayer. As a result, the fourth aspect of the present invention can bepreferably employed for a manufacturing (fabricating) process thatrequires the pre-bake or post-bake treatment (process).

Further, according to the fifth aspect of the present invention, inaddition to the above mentioned advantageous effects, the periodicpolarization reversal structure can be manufactured (fabricated) byapplying voltage to the crystal substrate by use of the conductivefluid. As a result, the manufacturing process can be simplified.

Yet further, according to the seventh aspect of the present invention,in addition to the above mentioned advantageous effects, the removingelectricity (neutralizing or destaticizing) can be carried out when thetemperature of the crystal substrate decreases where the unintentionaland random polarization reversal is more likely to occur (be generated).As a result, the effect of suppressing the polarization reversal can beobtained in a more assured manner.

These and other objects, aspects and advantages of the present inventionwill become apparent to a skilled person from the following detaileddescription when read and understood in conjunction with the appendedclaims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view useful to describe an exemplary perspectiveview of the quasi phase matching (QFM) type wavelength conversion devicefabricated (manufactured) by periodic inverse electric field setting;

FIG. 2A to FIG. 2D are a series of views useful to describe an exemplarymethod of fabricating the quasi phase matching (QFM) type wavelengthconversion device according to one embodiment of the present invention;

FIG. 3 is a schematic view useful to describe an exemplary frontcross-sectional view illustrating the removing electricity (neutralizingor destaticizing) step in the fabricating method according to oneembodiment of the present invention;

FIG. 4 is a schematic view useful to describe an exemplary frontcross-sectional view illustrating the removing electricity (neutralizingor destaticizing) step in a fabricating method according to anotherembodiment of the present invention;

FIG. 5A to FIG. 5C are a series of views useful to describe an exemplaryresult of the experiment for confirming the effect of suppressing thepolarization reversal by ion supply; and

FIG. 6 is a view useful to describe an exemplary result of the selectiveetching applied to the device in which the expected wavelengthconversion efficiency has not been obtained, and a picture of thesurface of the device on which the etching treatment is applied.

DETAILED DESCRIPTION OF THE INVENTION

Now, embodiments of the present invention (hereinafter referred to as“embodiments”) will be described with reference to the accompanyingdrawings in detail. Here, it should be noted that the present inventionis not limited to the illustrated and described embodiments, and theembodiments of the present invention are not limited to the illustratedand described embodiments.

The wavelength conversion device fabricated by the method according tothe embodiment is the quasi phase matching (QFM) type wavelengthconversion device as described above.

First Embodiment

FIG. 1 is a schematic view illustrating the quasi phase matching (QPM)type wavelength conversion device fabricated with the periodic inverseelectric field setting. As recognizable from FIG. 1, the term “periodic”means a periodically intermittent state or condition. In other words, itmeans that a region in which the inverse electric field is set and aregion in which the inverse electric field is not set are brought intoan alternatively successive state (in turn) when it is observed from thedirection perpendicular to z direction. In this case, the directionperpendicular to z axis is assumed as the incident direction of afundamental wave L_(ω).

In FIG. 1, the polarization structure of the quasi phase matching (QFM)type wavelength conversion device 8 is shown with an arrow directed, forexample, from −z side towards +z side. Although a boundary of regionseach of which is polarized in different direction each other is shown insolid line in FIG. 1, it should be noted that it does not mean an actualdevice includes such boundary line. Instead, the solid line in FIG. 1 isa line schematically drawn for better understanding of the polarizationstructure for illustrating purpose only.

As shown in FIG. 1, it is assumed that the polarizations areperiodically reversed and the reversal pitch (cycle) of the polarization(shown in FIG. 1 as Λ) is set to an adequate distance with respect towavelength of laser light (fundamental wave (harmonic)) Lω. Then, whenthe laser light Lω is propagated inside the device along x-y plane,phases of the second harmonics L₂ω generated in sequence arequasi-matched and are output from the device in a manner that the secondharmonics are not mutually attenuated but inversely strengthen eachother. Here, the polarization reversal pitch (cycle) A can be obtainedwith the following formula 1 with respect to the wavelength of thefundamental wave (harmonic)) Lω.

$\begin{matrix}{\Lambda = \frac{m\; \lambda}{2\left( {n_{2\; \omega} - n_{\omega}} \right)}} & \left( {{Formula}\mspace{14mu} 1} \right)\end{matrix}$

In the above formula 1, m denotes a number corresponding to an order(degree) of the harmonic (“1” in the present embodiment), λ denotes awavelength of the fundamental (reference) wave, n_(ω) denotes arefraction index at the wavelength of the fundamental wave, and n_(2ω)denotes a refraction index at the wavelength of the second harmonic. Forexample, when the wavelength of the fundamental wave λ is 1064 nm, andthe crystal is LiNbO₃, then the polarization reversal pitch (cycle) Λ iscalculated to be in the vicinity of (approximately) 7 μm.

FIG. 2 is a schematic view illustrating a fabrication method forfabricating this kind of quasi phase matching (QPM) type wavelengthconversion device according to the first embodiment. As described above,in order to fabricate the quasi phase matching (QPM) type wavelengthconversion device according to the first embodiment, it is necessary inprinciple to obtain good (favorable) quality non-linear optical crystalhaving uniform directions of natural polarizations and then to set theperiodic inverse electric field to reverse (inverse) polarizations.FIGS. 2A to 2D shows a sequence of procedures.

Setting periodic inverse electric fields may be achieved by causing theelectrode to directly contact a surface of the non-linear opticalcrystal and applying voltage to the surface of the non-linear opticalcrystal. At this moment, the polarization reversal pitch (cycle) A isextremely small in distance. Therefore a fine electrode structure has tobe formed. In addition, the formed electrode structure has to be removedultimately as is not required for the end (final) product.

Taken these requirements into consideration, the manufacturing methodaccording to the first embodiment employs a method of applying voltagewith conductive fluid to facilitate fabrication of the wavelengthconversion device.

More particularly, it is in principle necessary to obtain the non-linearoptical crystal for fabricating the quasi phase matching (QPM) typewavelength conversion device, and slice the non-linear optical crystalin prescribed thickness to allow the non-linear optical crystal to be ofplate like shape (hereinafter referred to as “crystal substrate”). Thecrystal substrate has natural polarizations in a uniform manner. Asrecognizable from FIG. 2, the crystal substrate may be obtained byslicing the non-linear optical crystal such that the thickness directionof the crystal substrate is z direction.

First, as shown in FIG. 2A, an insulating layer 5 is formed on at leastone plane (one surface) of the crystal substrate 4. In the firstembodiment, the insulating layer 5 is formed by coating the plane of thecrystal substrate 4 with the resist having insulating property. Althoughwhich plane is to be coated may be arbitrarily selected, FIG. 2Aillustrates an example of insulating layer 5 being coated on the planeat +z side. The resist to be coated is a photosensitive material, as itis subject to the photolithography treatment to allow patterns to beformed on the resist.

After the insulating layer 5 is formed, the insulating layer 5 issubject to the soft-bake (pre-bake) treatment (processing).Subsequently, patterns are formed on the insulating layer 5 by usingphotolithography technique (lithographic exposure or image development)to obtain an insulating layer pattern 6 (as shown in FIG. 2B). Theformed insulating layer pattern 6 is of a line-and-space like shape. Thepatterns on the insulating layer are formed in a manner that numerouslinear portions (ridge or elongated protrusion) extending in aprescribed direction are formed in parallel at a prescribed distance.

Next, after applying a hard-bake (post-bake) treatment (processing) tothe formed insulating layer pattern 6, voltage is applied to theinsulating layer pattern 6 with the conductive fluid 7 as shown in FIG.2C. In other words, as shown in FIG. 2C, the surface at + side of thecrystal substrate 4 is enclosed by a sealing ring 71 such as o shapedring, and the sealing ring 71 is closed (sealed) with a pad 72 providedwith an injection hole (pore) to form a closed space between the crystalsubstrate 4 and the pad 72. Subsequently, the conductive fluid 7 isinjected into the formed closed space to fill the space thereof. In thisstate, positive voltage is applied to the filled conductive fluid 7within the closed space at + side. On the other hand, the conductivefluid 7 filled within the other closed space at − side is set to anearth potential, or alternatively negative voltage is applied to thefilled conductive fluid 7. As an exemplary conducive fluid, for example,lithium chloride solution is in widespread use.

Applying voltage causes the electric field to be set to the crystalsubstrate 4 in the thickness direction (i.e., z direction). At +z side,positive voltage is applied only to the surface region that is notcovered (coated) with the insulating layer pattern 6. For this reason,the polarization in the surface region that is not covered (coated) withthe insulating layer pattern 6 is reversed, while in contrast, thepolarization in the region that is covered (coated) with the insulatinglayer pattern 6 is not reversed (i.e., the polarization in the region iskept in the direction of intrinsic (original) natural polarization.) Asa result, the periodic polarization reversal (periodicallypolarization-reversed) structure may be obtained.

After such periodic polarization reversal structure is formed, theinsulating layer pattern 6 is removed and necessary cleaning treatmentand an inspection process may be performed. Then the quasi phasematching (QPM) type wavelength conversion device 8 is completed (asshown in FIG. 2D). Although the fabricated device has the same exteriorappearance of the crystal substrate 4 as the pre-process crystalsubstrate 4, an internal polarization structure has a periodic structureas shown in FIG. 1.

During the above mentioned manufacturing (fabricating) process, theremay be a case that a plurality of quasi phase matching (QPM) devices 8are fabricated from a crystal substrate 4. In other words, afterprocesses shown in FIGS. 2A to 2D are carried out, there may be a casethat the crystal substrate 4 is cut at prescribed positions into aplurality of quasi phase matching (QPM) devices 8.

In the case that the quasi phase matching (QPM) type wavelengthconversion device 8 is fabricated according to the above mentionedmethod, as described above, it is turned out that the crystal substrate4 includes (has) the unintentional and random polarization reversal. Theinventor of the present invention has been conducting a research to theutmost on a new manufacturing process capable of suppressing the abovementioned unintentional and random polarization reversal. Consequentlyand ultimately, the inventor of the present invention has reached toconceive and reduction to practice an effective method capable ofsuppressing the unintentional and random polarization reversal. Itshould be noted that hereinafter and throughout the specification,suppressing the unintentional and random polarization is referred to asthe term “removing electricity”, “neutralizing” or “destaticizing”. Itis also referred to as the term “removing electricity on (from) asurface”, “neutralizing a surface” or “destaticizing a surface.”

More particularly, according to the method of the present embodiment,when (while) the change in temperature of the crystal substrate 4 isoccurring (occurs) during the process such as the post-bake or pre-baketreatment, ions are supplied to a surface of the crystal substrate 4 tosuppress the polarization reversal.

Now referring to FIG. 3, this mechanism will be described in detail.FIG. 3 is a front cross-sectional view illustrating the neutralizationin the manufacturing method according to the first embodiment.

As described above, the unintentional and random polarization reversaloccurs while the temperature of the crystal substrate 4 changes (ischanging). According to the inventor's research accomplished, inparticular, it is presumed that the unintentional and randompolarization reversal is more likely to occur when the temperature ofthe crystal substrate 4 decreases. It suggests that supplying ions (theion supply) is preferable to be carried out at such moment. According tothe first embodiment, during the pre-bake treatment (process) and/orpost-bake treatment (process), the crystal substrate 4 is left lying ina heating furnace 9 and then cooled (i.e., natural cooling). For thisreason, in the method according to the first embodiment, the heatingfurnace 9 is provided with an ionizer 91, which supplies ions to thecrystal substrate 4 during its temperature being decreasing in theheating furnace 9.

As shown in FIG. 3, in the first embodiment, the heating furnace 9 isprovided with a hot plate 93 and a covering member 94. The crystalsubstrate 4 is configured to be mounted on a tray (hot plate) 93 forheating and then be subject to be the heating process. The crystalsubstrate 4 is, after the heating by the tray 93 is stopped, naturallycooled on the tray 93. At this moment, the ionizer 91 supplies ions tothe surface of the crystal substrate 4. As shown in FIG. 3, the crystalsubstrate 4 mounted on the tray 93 is covered with the covering member94. The ionizer 91 is mounted (attached) to the covering member 94 sothat the ionizer 91 is configured to supply ions inside the coveringmember 94.

The ionizer 91 is configured to ionize air (for example, compressedair), and generate the same quantity of positive ion or ions (cation orcations) and negative ion or ions (anion or anions). As shown in FIG. 3,generated ions are irradiated (emitted) from the ionizer 91 and reach tothe surface of the crystal substrate 4.

At this moment, the crystal substrate 4 is naturally polarized (hasnatural polarization), and, in the example in FIG. 3, +z plane (i.e.,positive electric charge plane) is upwardly exposed. For this reason,among ions emitted from the ionizer 91, the negative ions aggregate(collect) together on the upper plane (+z plane) of the crystalsubstrate 4. Accordingly, even when the temperature of the crystalsubstrate 4 changes (is changing), the undesirable random polarizationreversal is suppressed to be generated. In other words, even in the casethat crystal defect or the like exists and therefore the polarizationreversal is about to occur (locally) at the position of the crystaldefect, such polarization reversal may be efficiently suppressed becausethe negative ions capture (moderate or suppress) the polarization at +zplane.

In general, the hot plate 93 is formed from a conductive material andgrounded. The positive ions and excess negative ions emitted from theionizer 93 flow into the earth (ground) through the hot plate 93.

Second Embodiment

Next, referring to FIG. 4, the other fabrication method according to asecond embodiment will be described below. FIG. 4 is a frontcross-sectional schematic view illustrating a manufacturing methodaccording to another embodiment of the present invention. In the secondembodiment shown in FIG. 4, an arrangement of the crystal substrate 4during the removal of electricity is different from the firstembodiment. That is, as shown in FIG. 4, the crystal substrate accordingto the second embodiment is arranged vertically.

More particularly, as shown in FIG. 4, the crystal substrate 4 arrangedin a vertically standing manner in the heating furnace 9. The crystalsubstrate 4 is hold by a substrate holding member 92 such that thevertically standing position (arrangement) of the crystal substrate 4 iskept. The substrate holding member 92 holds the crystal substrate 4 bothat an upper end and a lower end of the crystal substrate 4. Thus, themain plane of the crystal substrate 4 (i.e., a plate surface serving aseither +z plane or −z plane) becomes in an exposed state. In addition,the ionizer 91 is mounted at the upper section of the heating furnace 9,and the ionizer 91 supplies ions through an upper aperture of theheating furnace 9. As a direction of ion emission (irradiation) from theionizer 91 is a vertically downward direction, the main plane of thecrystal substrate 4 is in parallel to the direction of the ion emission.

As shown in FIG. 4, ions emitted from the ionizer 91 come down on thedown blow and reach the surfaces 41 and 42 of the crystal substrate 4.At this moment, the crystal substrate 4 has the natural polarization (isnaturally polarized), and one plane of the crystal substrate 4 serves asthe +z plane 41 (i.e., a positive electric charge plane) while the otherplace of the crystal substrate 4 serves as the −z plane 42 (i.e.,negative electric charge plane). Accordingly, among ions coming downfrom the ionizer 91, the positive ions flow to the −z plane 42 side tocollect on (in the vicinity of) the −z plane 42, while the negative ionsflow to the +z plane 41 to collect on (in the vicinity of) the +z plane41. As a result, the positive and negative ions collect on (in thevicinity of) the surfaces of the crystal substrate 4, respectively,according to the polarity of the natural polarization of the crystalsubstrate 4. Consequently, even when the temperature of the crystalsubstrate 4 changes (is changing), generation of the random polarizationreversal can be suppressed, as similar to the first embodiment.

FIG. 5 is a view illustrating an experimental result confirming aneffect of suppressing the polarization reversal by use of the ionsupply, as described above. In this experimental result, using theIonizer Model 306, manufactured by Hugle Electronics Inc., and thedistance of a region in which the random polarization reversal occurredwas observed with changing the ion irradiation level (amount).

This experiment employed the method according to the second embodimentshown in FIG. 4, using the circular crystal substrate 4 (LiNbO₃). Afterthe crystal substrate 4 has heated at 125 Celsius degree forapproximately two hours, the crystal substrate 4 was left lying in theheating furnace 9 until the temperature of the once heated crystalsubstrate 4 decreased to the room temperature. In FIG. 5, hatched linearea shows an area in which the unintentional and random polarizationreversal (i.e., the spot like polarization reversal as shown in FIG. 6)during the temperature drop (decrease).

In FIG. 5, the term “decay time” means a performance of the ionizer (theion supply quantity), and the shorter the decay time becomes, the largerthe ion supply level (amount or quantity) becomes. As shown in FIG. 5,as getting the decay time of the ionizer to be shorter (i.e., as gettingthe ion supply quantity to be larger), the area in which the randompolarization reversal occurs becomes smaller. The area in which therandom polarization does not occur extended such that the downward ionstream from upper side spread out the area. It is presumed that thelarger amount of ions collecting on the surface allows the area in whichthe random polarization reversal is suppressed to be expand.

As such, according to the fabrication method of the second embodiment,the unintentional and random polarization reversal may be suppressedwhen (while) the temperature of the crystal substrate changes (ischanging). Accordingly, higher quality quasi phase matching (QPM)wavelength conversion device may be fabricated that is capable ofaccomplishing higher conversion efficiency.

It should be noted that suppressing the polarization reversal by ionsupply on the surface has a significance that the polarization reversalcan be suppressed with a simplified configuration (structure) withleveraging the property of the ferroelectric crystal.

As a configuration suppressing the unintentional and random polarizationreversal, applying voltage to the crystal substrate with an electrodecontacting the crystal substrate may be conceivable. More particularly,the negative voltage is applied to an electrode plate contacting thecrystal substrate with covering +z plane thereof, and the positivevoltage is applied to another electrode plate contacting the crystalsubstrate with covering −z plane thereof. Although this kind ofconfiguration may be employable, it may entail the larger size ofconfiguration of the manufacturing equipment (i.e., the configurationfor applying voltage). It may also entail further consideration ofdefining the voltage to be applied in an appropriate manner.

In contrast, according to the second embodiment, employing the abovementioned configuration suppressing the polarization reversal with ionsupply does not entail the larger size of configuration of themanufacturing equipment (i.e., the configuration for applying voltage).Also, as the electric charge generated on the surface due to the naturalpolarization can be mitigated by ions and saturate naturally, easieradjusting and controlling the manufacturing process can be achieved.Here, trying to generally phrase the configuration for supplying ionsand applying voltage with the electrode plate, as applying voltage maybe perceived as a kind of electric charge supply, then the configurationaccording to the second embodiment may be understood as the electriccharge supply for suppressing (including preventing) the polarizationreversal.

Furthermore, according to the research result conducted by the inventorof the present invention, it is turned out that the above mentionedunintentional and random polarization reversal is more likely to occurin a phase when the temperature of the crystal substrate 4 decrease.Therefore, carrying out the process according to the second embodimentin this phase is turned out more effective. It is presumed that thereason why the random polarization reversal is more likely to occurduring the temperature decrease is that the niobium (Nb) ion and thelithium (Li) ion in the crystal are more likely to move for pursuing newstable state during the temperature decrease. In any event, when theabove mentioned operation for suppressing the polarization reversalduring the temperature decrease, the desirable effect may bedemonstrated in more assured manner.

As such, according to the research result conducted by the inventor ofthe present invention, it is turned out that the above mentionedunintentional and random polarization reversal is more likely to occurwhen the temperature changes more significantly. Thus, if the crystalsubstrate 4 is cooled after a heating process, then naturally coolingthe crystal substrate 4 is preferable. According to the experimentalresult conducted by the inventor of the present invention, thegeneration of the spot like pattern as shown in FIG. 5 was observed,even if when the crystal substrate 4 is left lying in the heatingfurnace 9 for a long time as described above. In order to allow thecrystal substrate 4 to decrease in temperature with taking furtherlonger time, cooling the crystal substrate 4 with heating (at weakenlevel of heating) may be conceivable. However, employing suchconfiguration may entail drastically degraded productivity. Avoidingsuch drawback is another advantageous effect of the method according tothe second embodiment.

As seen by comparing FIG. 3 and FIG. 4, while the first embodiment shownin FIG. 3 may suppress the polarization reversal only at one side planeof the crystal substrate 4, the second embodiment shown in FIG. 4 maysuppress the polarization reversal at both sides of the crystalsubstrate 4 by supplying reversed polarity ions. Accordingly, highereffect of suppressing the polarization reversal may be achieved.

Although in FIGS. 3 and 4 shows only one plate of the crystal substrate4, a plurality of crystal substrates 4 may be simultaneouslyheat-treated, and a plurality of crystal substrates 4 may beheat-treated while ions being supplied.

For example, in the first embodiment shown in FIG. 3, a plurality ofcrystal substrates 4 may be aligned on one tray 93, and may beheat-treated and simultaneously supplied with ions by being covered bythe covering member 94 including the ionizer 91. In this case, each ofthe crystal substrates 4 may be arranged such that z planes with samepolarity of the crystal substrates 4 face the upper side. Alternatively,it is preferable to arrange the crystal substrates 4 such that +z planesof half of crystal substrates 4 face the upper side and −z planes ofanother half of crystal substrates 4 face the upper side, because itallows the positive and negative ions can be equally utilized.

Likewise, also in another embodiment shown in FIG. 4, a plurality ofcrystal substrates 4 may be heat-treated with simultaneously beingsupplied with ions. For example, a substrate holding member 92 may be acassette-like member capable of accommodating a plurality of crystalsubstrates 4. The substrate holding member 92 holds each of the crystalsubstrates 4 such that the each of the crystal substrates 4 verticallystands and is mutually spaced each other. That is, the main plane ofeach of the crystal substrates 4 becomes exposed. Similarly, the ionizer91 is disposed at the upper portion of the heating furnace 9 and allowsemitted ions to reach the main plane(s) of each of the crystalsubstrates 4 to suppress the unintentional polarization reversal.

As described above, it should be noted that holding the crystalsubstrate 4 with the both main planes of the crystal substrates 4 beingexposed has a significance that the polarization reversal may besuppressed at both side by supplying the inverse polarity ions from bothside of the main planes of the crystal substrate 4. This significancemay be accomplished not only in the case that the crystal substrate 4 isvertically arranged but also in the case that the crystal substrate 4 isarranged differently, for example, horizontally. For example, thecrystal substrate 4 may be horizontally arranged, being hold by theholding member at both ends, and then treated (processed) in a hangingstate in the air (i.e., a state in which both upper and lower mainplanes of the crystal substrate 4 are exposed).

Furthermore, it should be noted that allowing the main plane of thecrystal substrate 4 to be in parallel to the emitting direction of ionsfrom the ionizer 91 has a significance that the ions may be uniformlyyet efficiently supplied to each area of the main planes. Thissignificance may be also accomplished in the case that the crystalsubstrate 4 is arranged horizontally with being hold in a hanging statein the air. It may be accomplished by arranging the ionizer 91 such thatthe ionizer 91 irradiates ions towards the horizontal direction.

In the above mentioned embodiments, the insulating layer 5 is configuredas a line-and-space shaped pattern 6, the conductive fluid contacts thesurface of the crystal substrate 4 that is not covered with theinsulating layer pattern 6, and the voltage is applied to the surface ofthe crystal substrate 4. However, alternatively, another embodiment maybe conceivable. In this another embodiment, the conductive layer may beformed on one plane of the crystal substrate 4, a line-and-space shapedpattern may be formed on the conductive layer by the photolithographytechnique, and then the voltage may be applied to the crystal substrate4 through the formed line-and-space shaped conductive pattern. When theinsulating layer 5 is configured as the line-and-space shaped pattern 6and the conductive fluid contacts the surface of the crystal substrate 4that is not covered with the insulating layer patter 6 to apply thevoltage to the crystal substrate 4, the manufacturing process may besimplified as it may eliminate the forming process of the conductivepattern and the removing process thereof.

In the above mentioned embodiments, the term “resist” is used to intendto mean that it allows the voltage not to be applied locally during thevoltage apply using the conductive fluid. Thus, the resist in the abovementioned embodiments is sufficient to function if it has requiredinsulation property, and having a tolerability against etchant is notnecessary. Nevertheless, the insulating material with tolerabilityagainst the etchant may be used for the material of the insulating layer5 according to the above mentioned embodiments.

The pre-bake treatment or post-bake treatment may be also applied to theresist formed during the photolithography for forming the conductivepattern. Generally, these kinds of bake treatments may be carried out inorder to improve the anti-etching property and also anti-plasma propertywhen the conductive layer is etched with the resist pattern serving as amask.

On the other hand, the pre-bake treatment and post-bake treatment in theabove mentioned embodiments may be carried out in order to improve theinsulation property during the voltage apply using the conductive fluid,or improve the adhesion property to the crystal substrate 4, or both,which is slightly different from the above mentioned general purpose.Accordingly, a condition for applying the pre-bake treatment to theinsulating layer 5 or the post-bake treatment to the insulating layerpattern 6 may be different from the condition in the case that theresist pattern is used as an etching mask in many cases.

Although the above mentioned embodiments are described with recitingLiNbO₃ as an exemplary non-linear optical crystal material constitutingthe device, other materials may be employed. For example, LiTaO₃ (PPLT),KTiOPO₄ (PPKTP) may be employed for fabricating the device by similarmethod.

Also, the quasi phase matching (QPM) wavelength conversion device isadvantageous in that arbitrarily desired coherent length Λ may beselected by adjusting the polarization reversal pitch (cycle) Λ.Accordingly, the above mentioned embodiments may be applied for thedevice converting the arbitral wavelength for the purpose other than inthe case of generating second harmonic (532 nm) from the infrared lightof 1064 nm as described above.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the present invention. The novel apparatuses and methodsthereof described herein may be embodied in a variety of other forms;furthermore, various omissions, substitutions and changes in the form ofthe apparatuses and methods thereof described herein may be made withoutdeparting from the gist of the present invention. The accompanyingclaims and their equivalents are intended to cover such forms ormodifications as would fall within the scope and gist of the presentinvention.

The present application is based upon and claims the benefit of apriority from Japanese Patent Application No. 2013-217660, filed on Oct.18, 2013, and the entire contents of which are incorporated herein byreference.

What is claimed is:
 1. A method of fabricating a wavelength conversiondevice, the device being fabricated from a crystal substrate and formedfrom a ferroelectric crystal demonstrating a non-linear optical effect,the method comprising: heating the crystal substrate; suppressing apolarization reversal on a surface of the crystal substrate in whichnatural polarization occurs when temperature of the crystal substratebeing changing due to the heating; and forming a structure that isperiodically polarization-reversed in the direction perpendicular to athickness direction of the crystal substrate on the surface of thecrystal substrate.
 2. The method of fabricating a wavelength conversiondevice according to claim 1, wherein the suppressing the polarizationreversal is carried out by collecting, on the surface of the crystalsubstrate, ions having a polarity different from a polarity on a regionof the surface of the crystal substrate in which natural polarizationoccurs, the surface being to be periodically polarization-reserved. 3.The method of fabricating a wavelength conversion device according toclaim 2, wherein the suppressing the polarization reversal is carriedout by use of an ionizer.
 4. The method of fabricating a wavelengthconversion device according to claim 1, further comprising: forming aninsulating layer on at least one plane of the crystal substrate; forminga pattern with line-and-space of the insulating layer; and periodicallyapplying voltage to the crystal substrate using the formed pattern withline-and-space of the insulating layer, wherein the heating is carriedout after the forming the insulating layer, and before or after theforming the pattern.
 5. The method of fabricating a wavelengthconversion device according to claim 2, further comprising: forming aninsulating layer on at least one plane of the crystal substrate; forminga pattern with line-and-space of the insulating layer; and periodicallyapplying voltage to the crystal substrate using the formed pattern withline-and-space of the insulating layer, wherein the heating is carriedout after the forming the insulating layer, and before or after theforming the pattern.
 6. The method of fabricating a wavelengthconversion device according to claim 3, further comprising: forming aninsulating layer on at least one plane of the crystal substrate; forminga pattern with line-and-space of the insulating layer; and periodicallyapplying voltage to the crystal substrate using the formed pattern withline-and-space of the insulating layer, wherein the heating is carriedout after the forming the insulating layer, and before or after theforming the pattern.
 7. The method of fabricating a wavelengthconversion device according to claim 4, the applying voltage furthercomprising: supplying conductive fluid to the crystal substrate in amanner that the conductive fluid contacts a region of the surface of thecrystal substrate that is not covered with the insulating layer, theregion consisting of each of linear portions constituting theline-and-space; and applying the voltage to the crystal substratethrough the conductive fluid.
 8. The method of fabricating a wavelengthconversion device according to claim 5, the applying voltage furthercomprising: supplying conductive fluid to the crystal substrate in amanner that the conductive fluid contacts a region of the surface of thecrystal substrate that is not covered with the insulating layer, theregion consisting of each of linear portions constituting theline-and-space; and applying the voltage to the crystal substratethrough the conductive fluid.
 9. The method of fabricating a wavelengthconversion device according to claim 6, the applying voltage furthercomprising: supplying conductive fluid to the crystal substrate in amanner that the conductive fluid contacts a region of the surface of thecrystal substrate that is not covered with the insulating layer, theregion consisting of each of linear portions constituting theline-and-space; and applying the voltage to the crystal substratethrough the conductive fluid.
 10. The method of fabricating a wavelengthconversion device according to claim 4, wherein the heating is carriedout after forming the insulating layer.
 11. The method of fabricating awavelength conversion device according to claim 5, wherein the heatingis carried out after forming the insulating layer.
 12. The method offabricating a wavelength conversion device according to claim 6, whereinthe heating is carried out after forming the insulating layer.
 13. Themethod of fabricating a wavelength conversion device according to claim1, wherein the suppressing the polarization reversal is carried out whenthe temperature of the crystal substrate decreases.
 14. The method offabricating a wavelength conversion device according to claim 2, whereinthe suppressing the polarization reversal is carried out when thetemperature of the crystal substrate decreases.
 15. The method offabricating a wavelength conversion device according to claim 3, whereinthe suppressing the polarization reversal is carried out when thetemperature of the crystal substrate decreases.
 16. The method offabricating a wavelength conversion device according to claim 4, whereinthe suppressing the polarization reversal is carried out when thetemperature of the crystal substrate decreases.
 17. The method offabricating a wavelength conversion device according to claim 5, whereinthe suppressing the polarization reversal is carried out when thetemperature of the crystal substrate decreases.
 18. The method offabricating a wavelength conversion device according to claim 6, whereinthe suppressing the polarization reversal is carried out when thetemperature of the crystal substrate decreases.
 19. The method offabricating a wavelength conversion device according to claim 7, whereinthe suppressing the polarization reversal is carried out when thetemperature of the crystal substrate decreases.
 20. A wavelengthconversion device fabricated by the method of fabricating a wavelengthconversion device according to claim 1.